Imaging while drilling “IWD” techniques for creating borehole images indicative of various borehole and formation characteristics are well known in oil drilling applications. For example, natural gamma ray, compensated density, photoelectric effect “PE”, inductive and galvanic resistivity, and caliper imaging techniques are well known. Such borehole imaging techniques are commonly utilized to provide a visual indication of the direction in which bed boundaries are crossed as well as to quantitatively estimate formation dip and strike angles. Borehole imaging techniques are also commonly utilized in geosteering operations.
Borehole images are commonly formed using data reduction techniques. For example, LWD images may be formed via binning or sectoring methodologies that group the data into a relatively small number of circumferential sectors about the periphery of the tool (e.g., 4, 8, and 16 sectors are commonly utilized). Such data averaging tends to advantageously reduce statistical variations in the raw data and reduces image size thereby sometimes enabling compressed images to be transmitted to the surface in real time while drilling (e.g., via conventional mud column telemetry techniques). However, data averaging also irretrievably destroys the high spatial frequency content of the image (e.g., the content pertaining to the fine geological structure of the formation). Consequently, image interpretation is often limited to an analysis of large-scale structural features.
Microresistivity imaging techniques (also referred to as galvanic resistivity) are sometimes used to obtain high resolution LWD images, for example, for detecting fractures and other fine features in the formation. However the use of non-conductive drilling fluid, or even the presence of a thin non-conductive film on the surface of the formation, can severely impede the flow of electrical current through the fluid into the formation and thereby significantly degrade image quality. As a result, acquisition of high resolution microresistivity LWD images is not always possible. But there remains a need for improved LWD imaging techniques for obtaining high-resolution images, particularly in nonconductive drilling fluid.